One common type of light sources is the fluorescent tube. It has many advantages, but suffers from serious drawbacks. For example, there is always a delay after the power has been turned on until it starts to operate giving full light. It needs complicated control equipment, which requires space. To obtain light with a source of this kind it is unfortunately necessary to use materials having negative environmental effects. It is for example a big disadvantage that mercury has to be used in this type of light sources.
Cathodolumninescent light sources is another interesting type of light sources. Such light sources, including an evacuated envelope containing a grid and a heated cathode, for emission of electrons, are known from GB, A, 2 070 849 (The General Electric Company Limited), GB, A, 2 097 181 (The General Electric Company PLC), GB, A, 2 126 006 (The General Electric Company plc) and GB, A, 2 089 561 (The General Electric Company Limited). Be insides of the envelopes are covered with a layer of phosphor of an electron-responsive type. These cathodoluminescent lamps have essentially the form of an electric bulb.
Since these light sources all have a heated cathode, the cathode has to be heated by special means, before the emission of light starts.
The use of electrons exciting phosphor to luminescence has the effect that more heat is produced than in comparable fluorescent tubes. It is therefore advantageous if the active surface, for the emission of light and for the necessary heat dissipation, is large. The cathodoluminescent lamps shown in the documents mentioned do not have optimal surfaces. To overcome the drawbacks and problems with the fluorescent tubes and cathodoluminescent light sources, light sources having field emission cathodes were developed.
A light source of this kind is disclosed in U.S. Pat. No. 5,588,893 (Kentucky Research and Investment Company Limited). A field emission cathode is arranged inside an evacuated glass container having a luminescent layer arranged on its inner surface. A modulator or extraction electrode is provided between the cathode and the luminescent layer. The cathode includes carbon fibers, arranged in bundles, preferably in a matrix, on a substrate. The content of U.S. Pat. No. 5,588,893 is incorporated herein by reference.
However in the last-mentioned known light source, electrons are emitted only in a direction perpendicular to the substrate. Also, there is no indication in the document how to produce the light source in a cost-efficient way.
The above mentioned U.S. Pat. No. 5,588,893 (Kentucky Research and Investment Company Limited) also discloses a field emission cathode of the kind mentioned above. The cathode disclosed includes carbon fibres, arranged in bundles, preferably in a matrix, on a substrate. The document also discloses a method including treatment of the emitting surfaces in order to achieve a cathode with higher efficiency than previous cathodes.
Further, WO, A1, 98/57344 (LightLab AB) and WO, A1, 98/57345 (LightLab AB) disclose light sources having cylindrical geometry and employing field emission. In order to obtain the necessary electric field for field emission, the mentioned light sources include grids or modulator electrodes arranged close to the field emitting surfaces of the cathodes. In those light sources a relatively high electric field has to be created between the cathode and the grid, and the distance between the field emitting surfaces and the grid has to be small and uniform in order to achieve a sufficient electric field for field emission and good distribution of electrons emitted from the cathode.
A further document, WO, A1, 97/07531 (Silzars et. al.) discloses a lighting apparatus including a field emission cathode. The cathode is built up of one or more fibers. The fibers are very thin, having a diameter less than 100 microns, and preferably less than 10 microns. The diameters are selected in order to achieve field emission at reasonable voltages. A construction according to this document having one fiber will be inoperative if the fiber is broken. Since the fiber is very thin, the probability of that it breaks appears to be high. However, the probability is probably somewhat lowered by arranging more than one fiber in parallel, for redundancy. Moreover, the electron emission surface is very small due to the small diameter of the fiber(s).
Previously known field emission cathodes are often of a complicated and fragile construction, especially as concerns the mountings and the attachment of field emitting bodies.
It has been found in connection with cathodes including standard carbon fibers and a grid that the electrical fields acting between the cathode and a grid or an anode can cause individual fibers to get loose from their carrier if they are not safely secured thereto. Once loose, the fibers will, in most cases, be attracted by the grid and cause a short circuit between the cathode and the grid, until it burns off after some time due to the resulting current through the fibres.